a. Field of the Invention
The present invention relates to pilot lights as used in monitoring industrial control circuits.
b. The Prior Art
It is common in electrical control circuits to have a large number of switches such as motor starters, limit switches, float switches, thermal switches and the like integrated into a complex circuit for controlling a line of machines or an industrial process. It is desirable to visually monitor the switching functions of the circuit, and for this purpose large numbers of pilot lights connected into the control circuit may be mounted on a control panel for visually displaying switch conditions by illuminating the respective pilot lights as associated switching takes place. Such pilot lights usually comprise individual units each with its own housing, lamp and terminals for connection into the control circuit. In some constructions a transformer also may be included in the housing to reduce the lamp voltage from the control circuit voltage.
The lamps of pilot lights are subject to failure, and burned out lamps can give false indications of circuit conditions by failing to illuminate when an associated switching function occurs. Thus, it is common to include a test circuit with the pilot lights to enable the simultaneous testing of an entire group of pilot lights by operation of a single test button. Pilot lights adapted for such testing may be called remote test pilot lights and they are characterized by the addition of test terminals and internal components that apply a test voltage to the lamp whenever the remotely located test button is operated. The internal components function to electrically isolate the test terminals from the normal signal terminals of the pilot light, so that voltages applied to one set of terminals do not feed back into the circuitry of the other set of terminals. However, the internal components that isolate the test terminals from the regular signal terminals are themselves subject to failure, and in some instances failures can cause malfunctions in the control circuits which the pilot lights monitor. For example, in some pilot lights diodes are inserted between a signal terminal and a test terminal to block voltages in the test circuit from feeding into the signal circuit being monitored, and to conversely block voltages in the monitored signal circuit from the test circuit. Such diodes may fail by shorting through their junctions, and the blocking function is then lost, with the result that voltages can be improperly fed between test and signal circuits. It thus becomes desirable to employ extra protective devices in circuit with the diodes to maintain isolation between the test and signals terminals.
Another consideration in pilot light design is the desirableness of using low voltage lamps, say of the order of 6.3 volts. The filaments of low voltage lamps are sturdier and can better withstand shock and vibration than the thinner filaments of higher voltage lamps. Also, low voltage lamps can safely be replaced by maintenance personnel without any necessity to deenergize the circuits to which the pilot lights are connected. To use low voltage lamps, it is usual practice to incorporate a transformer in a pilot light with the lamp connected across the low voltage secondary, but in pilot lights using diode isolation this is not practical for the reason the diodes would supply pulsating direct current to the transformer primary, making the transformer too inefficient for commercial practices. Hence, available diode type remote test pilot lights have not used low voltage lamps, except for special constructions in which a low voltage test circuit is connected to each pilot light through separate terminals, and the signal circuit voltage is fed to the lamp through a transformer with isolation diodes in the transformer secondary. Such construction requires an extra transformer and leads in the control circuit, which diminishes its acceptance in the industrial control industry.
In another form of commercial pilot light, the contacts of an electromechanical relay are placed between the signal terminal and the test terminal to isolate one from the other, and the relay coil is connected between the test terminal and a common line terminal. A relay allows for the insertion of a transformer with the primary being switched between the signal and test terminals, and the low voltage secondary being connected to the lamp. Relays, however, are subject to mechanical failure by reason of contact breakage or wear of parts, or such malfunctions as contact welding or opening of the operating coil circuit. It is desirable to eliminate reliance on such a mechanical operation, and an alternative use of properly tested solid state circuit components could provide long and reliable life.